Field of the Invention
The present invention relates to a fault detection system for detecting an abnormality of an actuator, based on a movement time of a movable member in the actuator.
Description of the Related Art
With an actuator in which a movable member is displaced upon supply of a pressure fluid (e.g., a pressurized gas), maintenance typically is carried out prior to the occurrence of a fault, when the frequency of use or a number of operations (operating time, duration of use) thereof reaches a given empirically determined level.
However, conventionally, even with a normal actuator that is not currently suffering from deterioration, under routine maintenance, replacement of the actuator may take place unnecessarily, leading to an increase in costs. Further, in the market, it has been sought to enhance productivity of equipment in which the actuator is used, and to reduce the cost of products made by such equipment, by shortening the stroke time (tact) and thereby increasing the tact time of the movable actuator. For this purpose, it is desirable for the maintenance interval not to be set based on the judgment of an operator, but rather for the actuator to be managed automatically and numerically.
Further, in general, it is thought that deterioration of an actuator in which a pressure fluid is used occurs due to load conditions applied to the actuator, or time-based variations, i.e., aging, of a fluid pressure device such as pneumatic device or the like in which the actuator is included. Furthermore, assuming that the occurrence of a fault in the actuator can be detected prior to a failure of the actuator due to deterioration caused by changes in the tact time, the fluid pressure device can continue to be used until just prior to the end of its life cycle, thereby enabling the equipment to be operated with maximum efficiency.
Thus, instead of carrying out maintenance operations based on frequency of use or a number of operations (operating time, duration of use), various types of fault detection systems, which are equipped with a failure prediction function for detecting a fault or abnormality of the actuator automatically and numerically, have been proposed.
For example, FIG. 9 illustrates a fault detection system 100 in which a fault of an actuator is detected by measuring variations in the flow rate and pressure of a pressure fluid. In such a fault detection system 100, a pressure fluid is supplied selectively to an actuator 106 from a fluid pressure source 102 through a directional switching valve 104. In the interior of the actuator 106, which comprises a cylinder, a piston 110 to which there is connected a piston rod 108 is displaced in left and right directions of FIG. 9 between one end 116 and another end 118 of the actuator 106.
The directional switching valve 104 comprises a 4-way 5-port single-acting solenoid valve having a solenoid 112 and a spring 114. More specifically, when the solenoid 112 is actuated by supplying an external control signal (operation command), the directional switching valve 104 supplies pressure fluid from the fluid pressure source 102 to the one end 116 of the actuator 106 through a port 120, whereas the fluid (pressure fluid) at the other end 118 of the actuator 106 is discharged to the exterior through a port 122. As a result, the piston 110 is displaced from the one end 116 to the other end 118.
On the other hand, when supply of the control signal is stopped, under operation of the spring 114, the directional switching valve 104 supplies the pressure fluid from the fluid pressure source 102 to the other end 118 through the port 122, whereas the pressure fluid at the one end 116 is discharged to the exterior through the port 120. As a result, the piston 110 is displaced from the other end 118 to the one end 116.
Further, at an intermediate location of tubes 123, 125 that serve to connect the directional switching valve 104 and the ports 120, 122, couplings 124, 126 are arranged respectively, which are constituted by parallel-connected throttle and check valves.
In this case, as shown by the dashed-line arrows in FIG. 9, there is a possibility of the pressure fluid to leak from respective portions of the fault detection system 100. More specifically, external leakage of pressure fluid can take place (1) from the respective tubes 123, 125, 127 that are arranged between the fluid pressure source 102, the directional switching valve 104 and the actuator 106, (2) from the directional switching valve 104, (3) from the piston rod 108 and from a non-illustrated packing provided between the cylinder and the piston rod 108, and (4) from the couplings 124, 126. Further, in the interior of the actuator 106 as well, there is a possibility for pressure fluid to leak between the one end 116 and the other end 118 via the piston 110 and a non-illustrated packing provided between the cylinder and the piston 110.
Thus, in the fault detection system 100, non-illustrated flow meters and pressure gauges are arranged in each of the tubes 123, 125, 127, whereby the flow rate of the pressure fluid is measured by the respective flow meters, and the pressure of the pressure fluid is measured by the respective pressure gauges. Consequently, since variations in the flow rate and pressure of the pressure fluid can be measured, faults at locations where the pressure fluid leaks can be detected, and replacement of the affected components prior to a failure thereof can be performed.
On the other hand, the fault detection system 130 of FIG. 10 detects an abnormality of an actuator 106 based on the stroke time of a piston 110. In addition to the respective constituent elements of the aforementioned fault detection system 100, the fault detection system 130 further includes a controller 132 such as a PLC (Programmable Logic Controller) or the like for supplying control signals to the solenoid 112 from an output 132a, a first sensor 134 arranged on the one end 116 of the actuator 106, and a second sensor 136 arranged on the other end 118 of the actuator 106. The couplings 124, 126 referred to above (see FIG. 9) are not provided in the fault detection system 130. Further, in the fault detection system 130, a silencer 138 is arranged in a discharge passageway for the pressure fluid, which is discharged from the one end 116 and the other end 118 of the actuator 106.
The first sensor 134 detects the piston 110 upon displacement thereof to the one end 116. The second sensor 136 detects the piston 110 upon displacement thereof to the other end 118. Detection signals indicative of detection results of the piston 110 by the first sensor 134 and the second sensor 136 are input to an input 132b of the controller 132. Thus, in the controller 132, a time (stroke time of the piston 110), from an output time at which the control signal is output to the directional switching valve 104, until an input time, at which the detection signal is input (i.e., a time at which displacement of the piston 110 is completed), is measured, and based on the measured stroke time, an abnormality of the actuator 106 is detected.
Further, similar to the fault detection system 130 of FIG. 10, techniques for detecting abnormalities of an actuator by using the stroke time of a movable member of the actuator are disclosed in Japanese Laid-Open Patent Publication No. 10-281113 (hereinafter referred to as Patent Document 1) and Japanese Laid-Open Patent Publication No. 2002-174358 (hereinafter referred to as Patent Document 2).
As disclosed in Patent Document 1, a velocity and a stroke time from initiation of an operation command of an actuator and a driven body are measured. The measured velocity and stroke time are compared with a reference velocity and stroke time during normal operation, and a judgment is made as to whether the actuator and the driven body are functioning normally.
As disclosed in Patent Document 2, a time from energization of a solenoid valve until the piston of a double-acting cylinder reaches a stroke end is measured as a stroke time, and a warning is issued if the measured stroke time becomes equal to or greater than a predetermined threshold value.